1. Field of the Invention
The present invention relates to interferometry, and more specifically, it relates to high accuracy diffraction interferometry.
2. Description of Related Art
Interferometry is the preferred method to measure the performance of optical elements and systems. In this method the wavefront of light reflected from or transmitted by the optic to be tested is interfered with the wavefront from a reference surface, to produce an interference fringe pattern. These interference fringes are then analyzed to ascertain the performance of the optic. For high performance imaging systems, such as those found in lithographic steppers used to make integrated circuits, this interferometric measurement must be made to ever increasing accuracy. The accuracy, however, is limited by how well the reference surface is characterized. Reference surfaces are typically no better than .lambda./50, where .lambda. is the wavelength of visible light, and thus are the limiting factor in fabricating higher performance optical systems. Therefore the fabrication of high accuracy optical systems, such as those needed for extreme ultraviolet projection lithography which require an accuracy of .lambda./1000, are impossible to qualify with confidence using existing interferometry.